Method and apparatus for in-situ monitoring of thickness during chemical-mechanical polishing

ABSTRACT

An apparatus and method for in-situ monitoring of thickness during chemical-mechanical polishing (CMP) of a substrate using a polishing tool and a film thickness monitor. The tool has an opening placed in it. The opening contains a monitoring window secured in it to create a monitoring channel. A film thickness monitor (comprising an ellipsometer, a beam profile reflectometer, or a stress pulse analyzer) views the substrate through the monitoring channel to provide an indication of the thickness of a film carried by the substrate. This information can be used to determine the end point of the CMP process, determine removal rate at any given circumference of a substrate, determine average removal rate across a substrate surface, determine removal rate variation across a substrate surface, and optimize removal rate and uniformity.

BACKGROUND OF THE INVENTION

The present invention relates to the field of semiconductor substrateprocessing and, more particularly, to the monitoring of material beingremoved during chemical-mechanical polishing of a semiconductorsubstrate.

The manufacture of an integrated circuit device requires the formationof various layers (both conductive, semiconductive, and non-conductive)above a base substrate to form necessary components and interconnects.During the manufacturing process, removal of a certain layer or portionsof a layer must be achieved in order to planarize or in order to formthe various components and interconnects. Chemical-mechanical polishing(CMP) is being extensively pursued to planarize a surface of asemiconductor substrate, such as a silicon substrate, at various stagesof integrated circuit processing. It is also used in polishing opticalsurfaces, metrology samples, micro-machinery, and various metal andsemiconductor based substrates.

CMP is a technique in which a polishing agent is used along with apolishing pad to polish away materials on a semiconductor substrate. Themechanical movement of the pad relative to the substrate, in combinationwith the chemical reaction of the polishing agent, provides an abrasiveforce with chemical erosion to planarize the exposed surface of thesubstrate (or a layer formed on the substrate).

In the most common method of performing CMP, a rotating wafer holdersupports a wafer, and a polishing pad rotates relative to the wafersurface. The wafer holder presses the wafer surface against thepolishing pad during the planarization process and rotates the waferabout a first axis relative to the polishing pad (see, for example, U.S.Pat. No. 5,329,732). The mechanical force for polishing is derived fromthe speed of the polishing pad rotating about a second axis differentfrom the first and the downward force of the wafer holder. A polishingagent is constantly transferred under the wafer holder, and rotation ofthe wafer holder aids in polishing agent delivery and averages out localvariations across the substrate surface. Since the polishing rateapplied to the wafer surface is proportional to the relative velocitybetween the substrate and the polishing pad, the polish rate at aselected point on the wafer surface depends upon the distance of theselected point from the two primary axes of rotation--that of the waferholder and that of the polish pad. This results in a non-uniformvelocity profile across the surface of the substrate, and therefore, ina non-uniform polish. Additionally, it is generally accepted by thoseexperienced in the art of CMP that a higher relative velocity betweenthe wafer and the polish pad is desired for superior planarizationperformance (see, for example, Stell et al., in "Advanced Metallizationfor Devices and Circuits--Science, Technology and Manufacturability" ed.S. P. Murarka, A. Katz, K. N. Tu and K. Maex, pg 151). However, a higheraverage relative velocity in this configuration leads to a lessdesirable velocity profile across the surface of the substrate, andtherefore, poor uniformity of polish.

This problem is solved by using a linear polisher. In a linear polisher,instead of a rotating pad, a belt moves a pad linearly across thesubstrate surface to provide a more uniform velocity profile across thesurface of the substrate. The substrate is still rotated for averagingout local variations as with a rotating polisher. Unlike rotatingpolishers, however, linear polishers result in a uniform polishing rateacross the substrate surface throughout the CMP process for uniformlypolishing the substrate.

Additionally, linear polishers are capable of using flexible belts, uponwhich the pad is disposed. This flexibility allows the belt to flex,which can cause a change in the pad pressure being exerted on thesubstrate. A fluid bearing formed by a stationary platen can be utilizedto control the pad pressure being exerted on a substrate at variouslocations along the substrate surface, thus controlling the profile ofthe polishing rate across the substrate surface.

Linear polishers are described in a patent application titled "Controlof Chemical-Mechanical Polishing Rate Across A Substrate Surface;" Ser.No. 08/638,464; filed Apr. 26, 1996 and in a patent application titled"Linear Polisher and Method for Semiconductor Wafer Planarization;" Ser.No. 08/759,172; filed Dec. 3, 1996. Fluid bearings are described in apatent application titled "Control Of Chemical-Mechanical Polishing RateAcross A Substrate Surface For A Linear Polisher;" Ser. No. 08/638,462;filed Apr. 26, 1996 and in U.S. Pat. No. 5,558,568.

Rotating CMP systems have been designed to incorporate various in-situmonitoring techniques. For example, U.S. Pat. No. 5,081,421 describes anin-situ monitoring technique where the detection is accomplished bymeans of capacitively measuring the thickness of the dielectric layer ona conductive substrate. U.S. Pat. Nos. 5,240,552 and 5,439,551 describetechniques where acoustic waves from the substrate are used to determineend point. U.S. Pat. No. 5,597,442 describes a technique where the endpoint is detected by monitoring the temperature of the polishing padwith an infrared temperature measuring device. U.S. Pat. No. 5,595,526describes a technique where a quantity approximately proportional to ashare of the total energy consumed by the polisher is used to determineend point. U.S. Pat. Nos. 5,413,941, 5,433,651 and European PatentApplication No. EP 0 738 561 A1 describe optical methods for determiningend point.

In U.S. Pat. No. 5,413,941, a laser light impinges onto an area of thesubstrate at an angle greater than 70° from a line normal to thesubstrate, the impinged laser light predominantly reflecting off thearea as opposed to transmitting through. The intensity of the reflectedlight is used as a measure of a change in degree of planarity of thesubstrate as a result of polishing. In U.S. Pat. No. 5,433,651, therotating polishing table has a window embedded in it, which is flushwith the table as opposed to the polishing pad. As the table rotates,the window passes over an in-situ monitor, which takes a reflectancemeasurement indicative of the end point of the polishing process. InEuropean Patent Application No. EP 0 738 561 A1, the rotating polishingtable has a window embedded in it, which, unlike the one in the '651patent, is flush with or formed from the polishing pad. A laserinterferometer is used as the window passes over an in-situ monitor todetermine the end point of the polishing process.

A linear polisher capable of in-situ monitoring for end point detectionusing a laser interferometer is described in U.S. patent applicationSer. No. 08/869,655, assigned to the assignee of the presentapplication.

Laser interferometry, however, has some inherent disadvantages. First,it measures absolute intensity of light emitting from an overlyingsubstrate layer, and is dependent upon the material being polished.Second, in laser interferometry the operator cannot directly determinewhether the film thickness being measured by the incident light isactually the desired finished thickness or some integer multiple thereofAdditionally, an inherent limitation of these end point detectionmonitoring systems is that one has to analyze the interference curve andfit it to a reasonable approximation. Thus, depending upon thewavelength used and the film properties, there is a finite amount ofremoval (2000-4000 Å) before the interference curve can be fitted to areasonable amount of accuracy. Further, using a single wavelength can,at best, only provide the removal rate, and based on the removal rateand prior knowledge of the initial thickness of the oxide, one canestimate the residual thickness of the oxide. Usually in a productionfab, the initial thickness of the dielectric varies within the controllimits of the deposition/growth process. Therefore, the assumption of aparticular initial thickness of oxide will create at least an errorequivalent to the natural (6 sigma) scatter of the deposition process.Further, the need for removing at least 2000-4000 Å before a reasonableestimate of the removal rate can be made can be difficult to implement,especially in multi-cluster tools where the process demands that eachcluster remove less than 2000 Å.

There is, accordingly, a need to provide thickness measurement in situwith CMP processes using either (i) platen-based systems such as thosethat rotate about their own axis, rotate in an orbital manner, oroscillate in a linear or circular manner, (ii) belt-based systems suchas those that use endless or non-endless belts, or (iii) oscillatingcarrier head systems to overcome the disadvantages found in the priorart.

SUMMARY OF THE INVENTION

This invention relates to chemical-mechanical polishing (CMP) of asubstrate using a polishing tool and a film thickness monitor forproviding a thickness of a substrate layer.

According to a first aspect of the invention, a polishing device has apolishing element having an opening placed in it and moving along apolishing path. A monitoring window is secured to the polishing elementto close the opening and to create a monitoring channel. A filmthickness monitor views a substrate through the monitoring channel toprovide an indication of a thickness of a film carried by the substrate.

According to a second aspect of the invention, the film thicknessmonitor comprises an ellipsometer, a beam profile reflectometer, or anoptical stress generator beam and monitoring probe.

According to a third aspect of the invention, the film thickness monitorcomprises a light source.

According to a fourth aspect of the invention, the moving meanscomprises a plurality of rollers operative to drive the polishingelement in a linear path past the substrate, a platen rotating about anaxis that passes through its center operative to drive the polishingelement in a curved path past the substrate, a platen rotating about anaxis that does not pass through its center operative to drive thepolishing element in a curved path past the substrate, or a platenmoving along a closed path operative to drive the polishing element in acurved path past the substrate.

According to a fifth aspect of the invention, the substrate carriermoves along a closed path.

According to a sixth aspect of the invention, the polishing element isused in a method for determining the thickness of a substrate layer.

According to a seventh aspect of the invention, a polishing element isused in a method for determining an end point of the CMP process byrepeatedly measuring film thickness of a substrate to determine whethera predefined thickness has been reached, in which case the fact that endpoint has been reached can be indicated and the CMP process can beterminated.

According to an eighth aspect of the invention, a polishing element isused in a method for determining removal rate at any given circumferenceof a substrate while performing CMP by determining the differencebetween two consecutive film thickness measurements made through thesame monitoring channel in the polishing element.

According to a ninth aspect of the invention, a polishing element isused in a method for determining average removal rate across a substratesurface while performing CMP by determining the average of thedifferences between at least two consecutive film thickness measurementstaken by at least two film thickness monitoring devices.

According to a tenth aspect of the invention, a polishing element isused in a method for determining removal rate variation across asubstrate surface while performing CMP by determining the variation ofthe differences between at least two consecutive film thicknessmeasurements taken by at least two film thickness monitoring devices.

According to an eleventh aspect of the invention, a polishing element isused in a method for optimizing the CMP process by characterizing apolishing process to determine effects of processing parameters; andthen determining removal rate and removal rate variation; and thenadjusting the polishing process parameters to optimize the removal rateand uniformity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a prior-art linear polisher.

FIG. 2 is a cross-sectional view of a linear polisher of a firstpreferred embodiment.

FIG. 3 is a plan view showing a placement of openings in a platen and apattern of openings on a belt to align with the openings in the platen.

FIG. 4 is a cross-sectional view of a fiber-optic transmission linedisposed between two layers of a belt to provide for an extended opticalsignal path from an outer surface of the belt to a first side surface ofthe belt.

FIG. 5 is a plan view showing a placement of sensing locations in abelt, but not in a platen, in which a fiber optic arrangement of FIG. 4is employed with multiple film thickness monitors.

FIG. 6 is a plan view showing a placement of sensing locations in abelt, but not in a platen, in which a fiber optic arrangement of FIG. 4is employed with only one film thickness monitor.

FIG. 7 is a schematic diagram of a rotating platen CMP device with afilm thickness monitor.

FIG. 8 is a schematic diagram of a film thickness monitor that includesan ellipsometer.

FIG. 9 is a plan view showing a placement of a plurality of openings ina platen and a belt having only one opening.

DETAILED DESCRIPTION OF THE PRESENTLY FIRST PREFERRED EMBODIMENTS

First Preferred Embodiment

Turning now to the drawings, FIG. 1 illustrates a prior art linearpolisher 100 utilized in planarizing a substrate (not shown) through atechnique generally known in the art as chemical-mechanical polishing(CMP). As shown in this figure, the linear polisher 100 has a substratecarrier 110 attached to a polishing head 105 that secures the substrate.The substrate is positioned on a belt 120, which moves about first andsecond rollers 130 and 135. As used herein, "belt" refers to aclosed-loop element comprising at least one layer, at least one layer isa layer of polishing material. A discussion of the layer(s) of the beltelement is developed below.

A polishing agent dispensing mechanism 140 provides a polishing agent150 on top of the belt 120. The polishing agent 150 moves under thesubstrate along with the belt 120 and may be in partial or completecontact with the substrate at any instant in time during the polishprocess. A platen 155 supports the belt 120 under the substrate carrier110.

Generally, the substrate carrier 110 rotates the substrate over the belt120. A mechanical retaining means, such as a retainer ring and/or avacuum typically holds the substrate in place.

The belt 120 is continuous and rotates about the rollers 130, 135.Driving means, such as a motor (not shown), rotate the rollers 130, 135,causing the belt 120 to move in a linear motion with respect to thesurface of the substrate.

As the belt 120 moves in a linear direction, the polishing agentdispensing mechanism 140 provides polishing agent 150 to the belt 120. Aconditioner (not shown) is typically used to recondition the belt 120during use by constantly scratching the belt 120 to remove polishingagent residue build-up and/or pad deformation.

The belt 120 moves between the platen 155 and the substrate, as shown inFIG. 1. A primary purpose of platen 155 is to provide a supportingplatform on the underside of the belt 120 to ensure that the belt 120makes sufficient contact with the substrate for uniform polishing.Typically, the substrate carrier 110 presses downward against the belt120 with appropriate force, so that the belt 120 makes sufficientcontact with the substrate for performing CMP. Since the belt 120 isflexible and will depress when the substrate presses downward onto it,the platen 155 provides a necessary counteracting support to thisdownward force.

The platen 155 can be a solid platform or it can be a fluid bearing(which includes one or more fluid channels). A fluid bearing ispreferred because the fluid flow from the platen 155 can be used tocontrol forces exerted against the underside of the belt 120. By suchfluid flow control, pressure variations exerted by the belt 120 on thesubstrate can be controlled to provide a more uniform polishing rate ofthe substrate. Examples of fluid bearings are disclosed in theaforementioned patent applications and in U.S. Pat. No. 5,558,568, eachincorporated by reference.

FIG. 2 shows a cross section of a first preferred embodiment of thepresent invention, which represents an improvement to the prior-artlinear polisher 100 described above. As in the prior-art embodiment, thelinear polisher 200 of FIG. 2 comprises a substrate carrier 210, a layerof polishing agent 215, a belt 220, and a platen 240 for performing CMPon a substrate (not shown). The belt 220 has a layer of polishingmaterial (not shown), an inner surface 201, and an outer surface 202.(The composition of the belt 220 is described in more detail below.) Newto this embodiment is an opening 230 in the belt 220 (extending from itsinner surface 201 to its outer surface 202) and an opening 245 in theplaten 240. Additionally, a layer of liquid mist such as that ofde-ionized water 255 lies between the belt 220 and the platen 240.

This embodiment performs CMP in the same way as the prior-art linearpolisher 100 described above. Unlike the above-described polisher 100,this polisher 200 can be used with an in-situ film thickness monitor250. Specifically, the openings 230, 245 in the belt 220 and the platen240 are used for in-situ monitoring of the substrate by the monitor 250.As the belt 220 moves linearly under the substrate during the CMPprocess, the opening 230 in the belt 220 passes over the opening 245 inthe platen 240. When the openings 230, 245 align (as shown in FIG. 2),an optical circuit is completed between the substrate and the filmthickness monitor 250, and insitu monitoring can be performed. Themonitoring process will be discussed in greater detail below.

While they can be left open, the openings 230, 245 in the belt 220 andthe platen 240 have monitoring windows 232, 242 embedded in them. Themonitoring window 232 in the belt 220 is substantially transparent tolight within a selected range of optical wavelengths and extendscompletely or partially between the inner 201 and outer 202 surfaces ofthe belt 220. Generally, the monitoring window 232 in the belt 220ensures that no polishing agent 215 or water leaks to the underside ofthe belt 220. By being flush with the outer surface 202 of the belt 220,implications with the polishing process are avoided. By being flush withthe inner surface 201 of the belt 220, the creation of turbulent regimesin the fluid bearing of the platen 240 is avoided (though, it could bejust a little raised or recessed).

Unlike the windows in the prior art, rotating platen systems, themonitoring window 232 should also be flexible enough to ride over therollers (which can range from 2 to 40 inches in diameter) moving thebelt 220 and should be made of a material that will have a minimumeffect on the polishing results due to its presence. Depending on themonitoring system used, the monitoring window 232 may also need aparticular optical characteristic (e.g., maximum transmission ofradiation from about 200 nm up to about 2000 nm in wavelength withminimum absorption or scattering).

The monitoring window 242 filling the opening 245 in the platen 240 ispreferably flush with the top surface of the platen 240 to preventpolishing agent from flowing into the film thickness monitor 250 and toavoid creation of turbulent regions in the fluid bearing of the platen240. As with the monitoring window 232 in the belt 220, the monitoringwindow 242 in the platen 240 preferably provides desired opticalcharacteristics (e.g., maximum transmission of the spectrum of lightbeing generated from the monitor 250 and reflected from the surface ofthe substrate).

Second Preferred Embodiment

While the belt 220 of the above embodiment contains only one opening, aplurality of openings can be used. As shown in FIG. 3, the belt 310 cancontain a plurality of openings 320, 322, 324, 326, 328. For eachopening 320, 322, 324, 326, 328 in the belt 310, there is acorresponding opening 330, 332, 334, 336, 338 in the platen under thesubstrate carrier 340. Each opening 330, 332, 334, 336, 338 is alignedwith a respective film thickness monitor. As above, each opening can beclosed by a monitoring window.

In this figure, there are five openings, one at the center of thesubstrate and four arranged at 90 degree intervals. It is appreciatedthat the number or the pattern of the openings is a design choice. Forexample, the openings can be linearly or concentrically arranged. Withseveral film thickness monitors distributed under respective locationsof the belt 310, non-uniformity of the polishing process across thesubstrate surface can be ascertained.

Alternatively, as FIG. 9 shows, a single opening 920 in the belt 910 canbe used with multiple openings 930, 932, 934 in the platen, each openingcorresponding to a respective film thickness monitor. As above, eachopening can be closed by a monitoring window. The openings 930, 932, 934in the platen are aligned in a straight line parallel to belt 910motion. When the belt opening 920 is aligned with one of the openings930, 932, 934 in the platen, the film thickness monitor corresponding tothat platen opening can make a measurement of the surface condition ofthe polished object. With this arrangement, the condition of multipleareas of the surface can be monitored with a single opening in the belt910. It is important to note that the number and position of platenopenings, as well as the number of straight lines parallel to the belt910, is a design choice.

Third Preferred Embodiment

FIG. 4 shows another alternative embodiment. Here, there is no openingin the platen for a monitoring channel. Instead, an opening 420 isformed in the belt 415 for an extended monitoring channel. This figureshows a belt 415 having two layers (one of which is layer 410), an innersurface 401, an outer surface 402, a first side surface 403, and asecond side surface 404. The monitoring channel 420 is such that theoptical path travels laterally parallel to the upper surface of onelayer 410 of the belt 415 from the outer surface 402 to the first sidesurface 403. A film thickness monitor 440 is positioned adjacent to thefirst side surface 403 of the belt 415, instead of under the belt 415.

In this embodiment, a monitoring window fills the opening 420 tocomplete the optical circuit from the substrate to the film thicknessmonitor 440. This monitoring window can be a flexible fiber opticelement.

As with the embodiments described above, this approach can beimplemented with more than one monitoring channel. FIG. 5 shows a planview of an embodiment having a plurality of monitoring channels 520,522, 524, 526, 528. Here, a linearly aligned, slanted hole pattern isshown formed on the belt 510. The distal end of the fiber-optictransmission lines are terminated adjacent to a row of film thicknessmonitors 530, 532, 534, 536, 538 arranged along the side of the linearlymoving belt 510. In this arrangement, the positions of the filmthickness monitors can be adjusted to align with the optic fibers, sincethe film thickness monitors can be made movable. Thus, this embodimentallows for less stringent requirement in the placement of the monitoringchannels, since adjustments can be made in the positions of the filmthickness monitors 530, 532, 534, 536, 538.

While a plurality of film thickness monitors are shown in FIG. 5, asingle film thickness monitor 630 can be used, as FIG. 6 illustrates.This single film thickness monitor 630 is positioned along side themoving belt 610 and takes the place of multiple film thickness monitors.In this embodiment, the optical-fiber-filled monitoring channels 620,622, 624, 626, 628 can be made to traverse across the film thicknessmonitor 630 in a linear arrangement. Although detection cannot beperformed simultaneously in multiple monitoring channels, as whenmultiple film thickness monitors are utilized, data can still beobtained for each monitoring channel.

It is important to note that in the above alternatives, the monitoringchannel can either extend from the outer to the first side surface (inwhich case the monitor can be positioned along the side of the belt) orextend from the outer surface to the inner surface of the belt (in whichcase the monitor can be at least partially disposed within the belt). Itis also important to note that the pattern of openings on the belt maybe repeated more than once in order to obtain multiple measurements perbelt rotation. This provides more data points per unit time, therebyimproving the quality of the results obtained.

In each of the embodiments described above that use a linear polishingelement, an optical circuit is completed during the polishing processwhen the monitoring channel in the belt is aligned with the filmthickness monitor, as detected by a sensor. The sensor is preferably ashort distance diffuse reflex sensor (such as Sunx model number CX-24).The sensor enables the film thickness monitor to measure the surfacestate of the substrate being polished. Unlike the sensors used in rotaryplaten systems described below, this sensor does not detect when a waferis aligned with a single monitoring channel in a moving platen, butrather detects when the monitoring channel in the belt is aligned withthe film thickness monitor.

Best Mode and Belt Construction

Using a fluid bearing (preferably air) is more advantageous than using asolid platen, since monitoring data can be used to adjust the fluidpressure at varying locations of the platen to provide in-situcorrection during the polishing process. It is preferred that the platenhave about 1-30 fluid flow channels. It is also preferred that a pre-wetlayer of de-ionized water mist be used between the platen and the beltto sweep away any polishing agent that happens to come underneath thebelt, preventing blockage of the flow channels.

The monitoring window in the platen is preferably made from a hard,scratch-resistant material such as sapphire. A sapphire window from theSwiss Jewel Company (Part No. W12.55) is preferred. The monitoringwindow in the platen is secured in place with an adhesive sufficientlystrong to withstand the conditions of the CMP process. It is preferredthat the monitoring window have an anti-reflection coating on one ormore surfaces.

In using the above embodiments, it is preferred that a carrier film suchas that available from Rodel (DF200) be used between the substrate andthe substrate carrier. The substrate carrier preferably presses thesubstrate against the belt with a pressure of about 5 psi.

The polishing agent has a pH of about 1.5 to about 12. One type ofpolishing agent that can be used is Klebesol available from Hoechst,although other types of polishing agent can be used depending on theapplication.

During the CMP process, the rollers preferably rotate at a rate so as toprovide a belt speed of about 400 ft/min. The belt should be tensionedwith a force of about 600 lbs.

As mentioned above, a "belt" comprises at least one layer of material,one of which is a layer of polishing material. There are several ways inwhich to construct a belt. One way uses a stainless steel belt, whichcan be purchased from Belt Technologies, having a width of about 14inches and a length of about 93.7 inches, inner diameter. (In additionto stainless steel, a base layer selected from the group consisting ofaramid, cotton, metal, metal alloys, or polymers can be used.) Thepreferred construction of this multi-layered belt is as follows.

The stainless steel belt is placed on the set of rollers of the CMPmachine and is put under about 2,000 lbs of tension. When the stainlesssteel belt is under tension, a layer of polishing material, preferablyRodel's IC 1000 polishing pad, is placed on the tensioned stainlesssteel belt. An underpad, preferably made of PVC, is attached to theunderside of the stainless steel belt with an adhesive capable ofwithstanding the conditions of the CMP process. The constructed beltpreferably will have a total thickness of about 90 mils: about 50 milsof which is the layer of polishing material, about 20 mils of which isthe stainless steel belt, and about 20 mils of which is the PVCunderpad.

There are several disadvantages of the above construction method. First,because the stainless steel belt needs to be tensioned on the rollers,there is down time for the CMP machine. Second, this constructionrequires technicians and time to place the pad on the stainless steelbelt.

To overcome these disadvantages, the belt can be formed as oneintegrated component as described in a patent application titled"Integrated Pad and Belt for Chemical Mechanical Polishing;" Serial No.Ser. No. 08/800,373, filed Feb. 14, 1997, hereby incorporated byreference. The preferred construction of such an assembly follows.

This belt is formed around a woven Kevlar fabric. It has been found thata 16/3 Kevlar, 1500 Denier fill and a 16/2 cotton, 650 Denier warpprovide the best weave characteristics. As is well known in the art,"fill" is yarn in the tension-bearing direction, and "warp" is yarn inthe direction perpendicular to the tension bearing direction. "Denier"defines the density and diameter of the mono-filament. The first numberrepresents the number of twists per inch, and the second number refersto the number of filaments that are twisted in an inch.

The woven fabric is placed in a mold that preferably has the samedimensions as the stainless steel belt described above. A clearpolyurethane resin (as described in more detail below) is poured intothe mold under a vacuum, and the assembly is then baked, de-molded,cured, and ground to the desired dimension. The resin may be mixed withfillers or abrasives in order to achieve desired material propertiesand/or polishing characteristics. Since fillers and abrasive particlesin the polishing layer may scratch the polished article, it is desiredthat their average particle size be less than about 100 microns. Such abelt can be obtained pre-constructed from Belting Industries.

Instead of molding and baking the woven fabric with polyurethane, alayer of polishing material, preferably a Rodel IC 1000 polishing pad,can be attached to the woven fabric or the preconstructed belt as it wason the stainless steel belt.

In any of these belt constructions, fillers and/or abrasive particles(having an average particle size less than 100 microns) can be dispersedthroughout the polishing layer to enable use of lower concentration ofabrasive particles in the polishing agent. The reduction of abrasiveparticle concentration in the polishing agent leads to substantial costsavings (typically, polishing agent costs represent 30-40% of the totalcost of CMP processes). It also leads to a reduction in light scatteringdue to the presence of polishing agent particles. This reduces noise inthe signal obtained by the monitor and helps in getting more accurateand repeatable results.

The polishing layer may also comprise a polishing agent transportchannel. The polishing agent transport channel is a texture or patternin the form of grooves (depressions) etched or molded into the surfaceof the polishing layer. These grooves may be, for example, ofrectangular, U-, or V-shape. Typically, these channels are less than 40mils deep, and less than 1 mm wide at the polishing layer's uppersurface. The polishing agent transport channels are typically arrangedin a pattern such that they run the length of the polishing surface.However, they may be arranged in any other pattern as well. The presenceof these channels greatly enhances the transport of polishing agentbetween the polishing layer and polish substrate. This leads to improvedpolishing rates and uniformity across the substrate surface.

With any of the belts described above, a hole may be punched in the beltat the desired location to form the opening. The opening in the belt ispreferably 1/2 inch in width (across the belt) by 31/2 inches in length(along the belt).

The monitoring window filling the opening in the belt can be made up ofa variety of materials such as clear polyurethane (solid, filled, blownor extruded), PVC, clear silicone, or many other plastics. It ispreferred, however, that clear polyurethane be used, as this materialhas maximum transmission of radiation from about 200 nm up to about 2000nm in wavelength with minimum absorption or scattering. A suitable clearurethane resin can be purchased as "Calthane ND 2300 System" and"Calthane ND 3200 System" from Cal Polymers, Inc., 2115 Gaylord St.,Long Beach, Calif. The layer of polishing material can be made from asimilar material to ensure minimum effect on the polishing results.

The monitoring window can be secured in the opening with an adhesivesufficiently strong to hold the monitoring window in place during theCMP process. The adhesive is preferably 2141 Rubber and Gasket adhesiveavailable from 3M, Minneapolis, Minn.

Alternatively, the monitoring window can be molded directly in the belt.For the belt with a stainless steel layer, the polyurethane resin can becast in the opening. A casting with a mirror-finished rubber lining canbe placed on both sides of the opening during the curing process. Forthe belt with the woven fabric layer, openings can be made in the wovenfabric before placing it in the mold. After the baking process describedabove, the opening in the belt would contain the polyurethane monitoringwindow.

As an alternative to placing openings in the belt, each layer of thebelt can be partially or completely made of a material substantiallytransparent to light within a selected range of optical wavelengths,such as about 200 nm to about 2000 nm, eliminating the need to provide amonitoring window in the belt. For example, the fabric may be woven withKevlar or some other material so as to provide openings in the fabric,or may be constructed with optically clear fiber. Clear polyurethane (orsome other clear material) is then molded onto the fabric in a mannerdescribed above. This results in a belt assembly that is appropriate forfilm thickness measurements.

Fourth Preferred Embodiment

FIG. 7 illustrates a fourth preferred embodiment. In this embodiment, arotating polishing device 700 is used for CMP instead of a linear belt.Such an apparatus is well known in the art (U.S. Pat. Nos. 5,329,732;5,081,796; 5,433,651; 4,193,226; 4,811,522; and 3,841,031, herebyincorporated by reference).

As shown in FIG. 7, a rotating wafer holder 720 supports a wafer, and apolishing element (a polishing pad 730 on a platen 712) rotates relativeto the wafer surface. The wafer holder 720 presses the wafer surfaceagainst the polishing pad 730 during the planarization process androtates the wafer about a first axis 710 relative to the polishing pad730 (see, for example, U.S. Pat. No. 5,329,732). The polishing pad 730is typically a relatively soft wetted material such as blownpolyurethane and it, with the platen 712, rotates around an axis 715(unlike the stationary platen used with the linear belt).

The mechanical force for polishing is derived from the speed of thepolishing pad 730 rotating about a second axis 715 different from thefirst 710 and the downward force of the wafer holder 720. A polishingagent (per the specifics described above for the linear polishing tool)is constantly transferred under the wafer holder 720, and rotation ofthe wafer holder 720 aids in polishing agent delivery.

Since the polishing rate applied to the wafer surface is proportional tothe relative velocity between the substrate and the polishing pad 730,the polish rate at a selected point on the wafer surface depends uponthe distance of the selected point from the two primary axes ofrotation--that of the wafer holder 720 and that of the polish pad 730.This results in a non-uniform velocity profile across the surface of thesubstrate, and therefore, in a non-uniform polish.

In situ monitoring can take place with such an apparatus by providing anopening 740 in the rotating platen 712, in the polishing pad 730, orboth. A monitoring window secures to the polishing element to close theopening in at least the platen 712, creating a monitoring channel in thepolishing element. A film thickness monitor 750 is disposed under theopening 740 at certain times during the angular rotation of the platen712 and polishing pad 730. (The use of the monitor 750 is described inmore detail below.) It is important to note that multiple openings,monitoring windows, and film thickness monitors can be used.

In addition to a platen rotating about an axis that passes through itscenter, a platen rotating about an axis that does not pass through itscenter can be used to drive the polishing element in a curved path pastthe substrate. Additionally, the platen can move along a closed path todrive the polishing element in a curved path past the substrate. (SeeParikh et al., "Oxide CMP on High-Throughput Orbital Polisher," Feb.13-14, 1997 CMP-MIC Conference and WO 96/36459.) Also, in any of the CMPsystems described above, the substrate carrier can move along a closedpath.

Film Thickness Monitors

The film thickness monitor, mentioned above, can be used to calculatethe thickness of a layer on a substrate. The following is a discussionof three types of thickness monitoring techniques.

Ellipsometry

The thickness of a film of a substrate can be calculated by usingellipsometry, as described in U.S. Pat. Nos. 5,166,752, 5,061,072,5,042,951, 4,957,368, 4,681,450, 4,653,924, 4,647,207 and 4,516,855,each of which is hereby incorporated by reference.

FIG. 8 shows the components of a system 800 using ellipsometry. Thesystem 800 comprises a light source 820, a beam property selector 815, abeam former 810, a beam receiver 825, a reflected beam analyzer 830, anda data processor 835. This system 800 is used to calculate the thicknessof a film on a substrate 890 positioned on a CMP tool 895 (e.g., arotating belt or a linear belt) as described below.

The light source 820 generates optical radiation, which is polarized bythe beam property selector 815. The beam former 810 focuses thepolarized light beam on a substrate 890. As shown in FIG. 8, thepolarized light beam passes through a window (i.e., the monitoringchannel) 893 of the CMP tool 895. The beam receiver 825 captures thepolarized light beam reflected by the substrate 890. The reflected beamanalyzer 830 measures polarization changes in the beam associated withreflectance from the substrate.

Such polarization changes, which may include both amplitude and phasechanges, are sensitive to the thickness and optical properties of thefilm on the substrate 890. It is from these changes that the dataprocessor 835 calculates the thickness of the film on the substrate 890.

Ellipsometry typically uses oblique illumination, i.e. the angle ofincidence Θ between the incident light beam and the normal to thesubstrate is preferably greater than zero. The angle between a reflectedlight beam and the normal is also equal to the angle of incidence Θ. Theangle of incidence Θ should be close to the Brewster angle of the film.In practice, the preferred angle of incidence Θ ranges from 45° to 70°.Ellipsometry is well suited for monitoring film thickness, even for thinfilms having a thickness in the range of 0-100 Å.

Beam Profile Reflectometry

In another monitoring system, the thickness of a film of a substrate iscalculated by using a beam profile reflectometer, as known in the art asmultiple-angle illumination. In this system, the intensity profile of areflected beam is measured, and the S- and P-polarization reflectivitiesof a sample are simultaneously obtained over a wide range of angles.Such a system is described in "Multiparameter Measurements of Thin FilmsUsing Beam-Profile Reflectometry," J. Appl. Phys. Vol. 73 No. 117035-7040 (Jun. 1, 1993), which contains additional referencesconcerning this system.

Stress Pulse Analyzer

In another monitoring system, the thickness of a film is obtained usinga system that generates stress pulses (ultrasound waves) by means ofshort optical pulses (pump beam). By monitoring the stress pulse orultrasound wave with a probe beam and analyzing the propagationthroughout the film or film stack, the film thickness can be determined.Such a system is described in U.S. Pat. No. 4,710,030 (herebyincorporated by reference) and in "Picosecond Ultrasonics," IEEE Journalof Quantum Electronics, Vol. 25, #12 p.2562 (December 1989).

It is important to note that the above film thickness monitors aremerely examples and that other techniques that provide thickness can beused. For example, thickness can be preferably measured withmulti-wavelength spectroscopy (as described in U.S. patent applicationSer. No. 08/863,644, assigned to the assignee of the presentapplication, hereby incorporated by reference).

The above embodiments can be used in a method for determining thethickness of a substrate layer during the CMP process. First, asubstrate carrier would hold a substrate against a linearly moving beltor a rotating platen, either having a monitoring channel (as describedabove) and being wetted with a polishing agent. When the monitoringchannel in the belt or rotating platen aligns with the film thicknessmonitor, thickness of the layer of the substrate can be determined byusing ellipsometry, beam profile reflectometry, or a stress pulseanalyzer.

Information regarding thickness has several uses. For example, it isimportant to stop the CMP process upon the removal of the last unwantedlayer. As a consequence, end point detection is necessary and highlydesirable when the last layer has been removed. End point detection canbe determined by the thickness of the substrate layer. With thisinformation, the CMP process can automatically or manually end.

Specifically, when a monitoring channel in the CMP tool aligns with afilm thickness monitor, an optical circuit is completed between the filmthickness monitor and the substrate. This enables measurement of thesurface state of the substrate. Each time a monitoring channel in theCMP tool is aligned with a film thickness monitor, a film thicknessmeasurement is made, resulting in a sequence of film thicknessmeasurements during the CMP process. Therefore, the film thicknessmonitors described above may be used to determine and indicate the endpoint and may be used to manually or automatically end the CMP processin the above-described embodiments.

Thickness information can also be used in a method for determiningremoval rate at any given circumference of a substrate while performinga chemical-mechanical polishing process. When a monitoring channel inthe CMP tool aligns with a film thickness monitor, the film thicknessmonitor determines film thickness at any given circumference on thesubstrate as described above. The difference of two consecutive filmthickness measurements made through the same monitoring channel in theCMP tool is the film removal rate per polishing element revolution.Therefore, for a known tool speed, removal rate of the substrate isdetermined as thickness per unit time.

This method can also be adapted to determine the removal rate variationand average removal rate across a substrate surface. This is achieved ina similar manner to that described above, but with the use of multiplemonitoring channels in CMP tool. In this case, each monitoring channelresults in a film thickness measurement at a predefined circumference ofthe wafer substrate. Therefore, with every polishing element revolution,multiple film thickness measurements are made across the substratesurface. As described above, each of the measurements is converted intoa removal rate. The average and variation of the removal rate across thesubstrate surface is thus computed. For example, the standard deviationof the measurements is indicative of the removal rate variation acrossthe substrate surface.

Additionally, information regarding thickness can be used to adjustprocessing parameters of the CMP device. Removing uniformity can changeduring polishing of a substrate as a result of changes in belt (ormoving platen) and substrate carrier conditions. With the film thicknessmonitors described above, the thickness of a substrate layer can be usedto determine whether, for example, the center of the substrate is beingpolished at the same rate as the edge of the substrate. With thisinformation, the polishing tool parameters can be modified, eithermanually or automatically, to compensate for the detectednon-uniformity.

More specifically, the polishing process is first characterized in orderto determine the effects of polish parameters such as polish pressure,belt or platen speed, carrier speed, polishing agent flow, etc. onresponses such as substrate removal rate, uniformity, etc. A suitablemodel may be generated using software such as RS/1 available from BBNSoftware. During the polishing process, removal rate and removal ratevariation across the substrate (uniformity) are determined as describedabove. This information would then be used in conjunction with the modeldeveloped to adjust the polish parameters (such as, but not limited to,down force, tool speed, and carrier speed) in order to optimize theremoval rate and/or uniformity. This optimization may happen either inreal time or in a delayed manner.

It is important to note that while "substrate" has been used as aworking example for the above-described embodiments, any polished objectcan be used.

It is intended that the foregoing detailed description be understood asan illustration of selected forms that the invention can take and not asa definition of the invention. It is only the following claims,including all equivalents, which are intended to define the scope ofthis invention.

What is claimed is:
 1. In a chemical mechanical polishing device of thetype comprising: least two rollers, a belt comprising a layer ofpolishing material and mounted to extend between the rollers such thatrotation of the rollers drives the belt, and a substrate carrierpositioned adjacent the belt to press a substrate against the beltduring a polishing operation; the improvement comprising:the belt havingat least one opening formed therein, the opening positioned to move intointermittent alignment with the substrate during the polishingoperation; the belt further comprising a monitoring window secured tothe belt to close the opening and to create a monitoring channel in thebelt, the window comprising a flexible material adapted to flex with thebelt as the window moves around and between the rollers in use; and saiddevice further comprising a film thickness monitor, said film thicknessmonitor comprising an ellipsometer responsive to optical radiationreflected from the substrate through the monitoring channel during thepolishing operation to provide an indication of thickness of a filmcarried by the substrate.
 2. In a chemical mechanical polishing deviceof the type comprising: at least two rollers, a belt comprising a layerof polishing material and mounted to extend between the rollers suchthat rotation of the rollers drives the belt, and a substrate carrierpositioned adjacent the belt to press a substrate against the beltduring a polishing operation; the improvement comprising:the belt havingat least one opening formed therein, the opening positioned to move intointermittent alignment with the substrate during the polishingoperation; the belt further comprising a monitoring window secured tothe belt to close the opening and to create a monitoring channel in thebelt, the window comprising a flexible material adapted to flex with thebelt as the window moves around and between the rollers in use; and saiddevice further comprising a film thickness monitor, said film thicknessmonitor comprising a beam profile reflectometer responsive to opticalradiation reflected from the substrate through the monitoring channelduring the polishing operation to provide an indication of thickness ofa film carried by the substrate.
 3. In a chemical mechanical polishingdevice of the type comprising: at least two rollers, a belt comprising alayer of polishing material and mounted to extend between the rollerssuch that rotation of the rollers drives the belt, and a substratecarrier positioned adjacent the belt to press a substrate against thebelt during a polishing operation; the improvement comprising:the belthaving at least one opening formed therein, the opening positioned tomove into intermittent alignment with the substrate during the polishingoperation; the belt further comprising a monitoring window secured tothe belt to close the opening and to create a monitoring channel in thebelt, the window comprising a flexible material adapted to flex with thebelt as the window moves around and between the rollers in use; and saiddevice further comprising a film thickness monitor, said film thicknessmonitor comprising an optical stress generator beam and monitoring probebeam responsive to reflected probe beam radiation from the substratethrough the monitoring channel during the polishing operation to providean indication of thickness of a film carried by the substrate.
 4. Theinvention of claim 1, 2, or 3, wherein the film thickness monitorcomprises a light source operative to illuminate the substrate via themonitoring channel with optical radiation during the polishingoperation.
 5. The invention of claim 1, 2, or 3, wherein the filmthickness monitor further comprises a sensor for detecting when themonitoring window in the belt is in alignment with the film thicknessmonitor.
 6. A method for determining an end point of achemical-mechanical polishing process, the method comprising:(a)providing a belt having an opening formed therein and comprising amonitoring window secured to the belt to close the opening and to createa monitoring channel in the belt the window comprising a flexiblematerial adapted to flex with the belt as the belt moves in use; (b)measuring a film thickness of a substrate during a chemical-mechanicalpolishing process when the monitoring channel aligns with a filmthickness monitor; and (c) indicating that end point has been reached inresponse to the measured film thickness being at a redefined thickness.7. The invention of claim 6, wherein the film thickness is measured byan ellipsometer.
 8. The invention of claim 6, wherein the film thicknessis measured by a beam profile reflectometer.
 9. The invention of claim6, wherein the film thickness is measured by a stress pulse analyzer.10. The invention of claim 6 further comprising the step of terminatingthe chemical-mechanical polishing process when the film thicknessreaches a predefined thickness.
 11. A method for determining removalrate per polishing element revolution while performing achemical-mechanical polishing process, the method comprising:(a)providing a belt having an opening formed therein and comprising amonitoring window secured to the belt to close the opening and to createa monitoring channel in the belt, the window comprising a flexiblematerial adapted to flex with the belt as the belt moves in use; (b)measuring a first film thickness of a substrate during achemical-mechanical polishing process when the monitoring channel alignswith a film thickness monitor; (c) measuring a second film thickness ofa substrate during the chemical-mechanical polishing process when themonitoring channel realigns with the film thickness monitor; and (d)determining removal rate by calculating a difference between the secondfilm thickness and the first film thickness.
 12. The invention of claim11, wherein the first film thickness and the second film thickness aremeasured by an ellipsometer.
 13. The invention of claim 11, wherein thefirst film thickness and the second film thickness are measured by abeam profile reflectometer.
 14. The invention of claim 11, wherein thefirst film thickness and the second film thickness are measured by astress pulse analyzer.
 15. A method for determining average removal rateper polishing element revolution across a substrate surface whileperforming a chemical-mechanical polishing process, the methodcomprising:(a) providing a belt having a plurality of openings formedtherein and comprising a plurality of monitoring windows secured to thebelt to close the respective openings and to create respectivemonitoring channels in the belt, the windows comprising a flexiblematerial adapted to flex with the belt as the belt moves in use; (b)measuring a plurality of film thicknesses of a substrate during achemical-mechanical polishing process, each of the plurality of filmthicknesses being measured when one of the plurality of the monitoringchannels aligns with a film thickness monitor; and (c) determiningaverage removal rate by calculating an average of differences betweenthe measured plurality of film thickness.
 16. The invention of claim 15,wherein the plurality of film thicknesses are measured by anellipsometer.
 17. The invention of claim 15, wherein the plurality offilm thicknesses are measured by a beam profile reflectometer.
 18. Theinvention of claim 15, wherein the plurality of film thicknesses aremeasured by a stress pulse analyzer.
 19. A method for determiningremoval rate variation across a substrate surface while performing achemical-mechanical polishing process, the method comprising:(a)providing a belt having a plurality of openings formed therein andcomprising a plurality of monitoring windows secured to the belt toclose the respective openings and to create respective monitoringchannels in the belt, the windows comprising a flexible material adaptedto flex with the belt as the belt moves in use; (b) measuring aplurality of film thicknesses of a substrate during achemical-mechanical polishing process, each of the plurality of filmthicknesses being measured when one of the plurality of the monitoringchannels aligns with a film thickness monitor; and (c) determiningremoval rate variation by calculating a variation of differences betweenthe measured plurality of film thickness.
 20. The invention of claim 19,wherein the plurality of film thicknesses are measured by anellipsometer.
 21. The invention of claim 19, wherein the plurality offilm thicknesses are measured by a beam profile reflectometer.
 22. Theinvention of claim 19, wherein the plurality of film thicknesses aremeasured by a stress pulse analyzer.
 23. A method of optimizing achemical-mechanical polishing process comprising:(a) providing a belthaving an opening formed therein and comprising a monitoring windowsecured to the belt to close the opening and to create a monitoringchannel in the belt, the window comprising a flexible material adaptedto flex with the belt as the belt moves in use; (b) measuring aplurality of film thicknesses of a substrate during achemical-mechanical polishing process when the monitoring channel alignswith a film thickness monitor; (c) determining removal rate with themeasured plurality of film thicknesses; and then (d) adjusting apolishing process parameter to optimize the removal rate.
 24. Theinvention of claim 23 further comprising:(e) determining removal ratevariation; and then (f) adjusting a polishing process parameter tooptimize uniformity.
 25. A method for determining a thickness of a layeron a substrate during chemical-mechanical polishing, the methodcomprising:(a) providing a belt having an opening formed therein andcomprising a monitoring window secured to the belt to close the openingand to create a monitoring channel in the belt, the window comprising aflexible material adapted to flex with the belt as the belt moves inuse; (b) performing chemical-mechanical polishing on a substrate byholding the substrate in a substrate carrier against the belt; and (c)using an ellipsometer responsive to optical radiation reflected from thesubstrate through the monitoring channel during the polishing operationto provide an indication of thickness of a film carried by thesubstrate.
 26. A method for determining a thickness of a layer on asubstrate during chemical-mechanical polishing, the methodcomprising:(a) providing a belt having an opening formed therein andcomprising a monitoring window secured to the belt to close the openingand to create a monitoring channel in the belt, the window comprising aflexible material adapted to flex with the belt as the belt moves inuse; (b) performing chemical-mechanical polishing on a substrate byholding the substrate in a substrate carrier against the belt; and (c)using a beam profile reflectometer responsive to optical radiationreflected from the substrate through the monitoring channel during thepolishing operation to provide an indication of thickness of a filmcarried by the substrate.
 27. A method for determining a thickness of alayer on a substrate during chemical-mechanical polishing, the methodcomprising:(a) providing a belt having an opening formed therein andcomprising a monitoring window secured to the belt to close the openingand to create a monitoring channel in the belt, the window comprising aflexible material adapted to flex with the belt as the belt moves inuse; (b) performing chemical-mechanical polishing on a substrate byholding the substrate in a substrate carrier against the belt; and (c)using an optical stress generator beam and monitoring probe beamresponsive to reflected probe beam radiation from the substrate throughthe monitoring channel during the polishing operation to provide anindication of thickness of a film carried by the substrate.